J.E. Lees

The operation of microchannel plates at high count rates

Abstract The measured count rate characteristics of a large number of microchannel plate electron multipliers are compared with the predictions of a universal, paralysable-counter model. Individual plate resistances for the experimental study lie in the range 27–2450 MΩ. The gain behaviour of single channel plates is shown to differ from that of multi-stage MCP detectors. The measured dependence of multi-stage multiplier recovery time on illuminated area is interpreted in terms of inter-channel coupling and of changes in conduction current during plate operation. Our findings have significance for the calibration of particle and photon spectrometers and for the development of future channel plate detectors with extended dynamic range.

AXAF High-Resolution Camera (HRC): calibration and recalibration at XRCF and beyond

The high resolution camera (HRC) is a microchannel plate based imaging detector for the Advanced X-Ray Astrophysics Facility (AXAF) that will be placed in a high earth orbit scheduled for launch in August, 1998. An end-to-end calibration of the HRC and the AXAF high resolution mirror assembly (HRMA) was carried out at the Marshall Space Flight Centers X-Ray Calibration Facility (XRCF). This activity was followed by several modifications to the HRC to improve its performance, and a series of flat field calibrations. In this paper, and the following companion papers, we discuss the calibration plans, sequences, and results of these tests. At the time of this conference, the HRC has been fully flight qualified and is being integrated into the science instrument module (SIM) in preparation for integration into the AXAF spacecraft.

The Microsphere Plate: a new type of electron multiplier

Abstract Microsphere Plates (MSPs), a new type of electron multiplier, consisting of sintered disks of glass beads, have recently become available from El-Mul Technologies Ltd. The principles of MSP operation are similar to those of microchannel plates (MCPs). We present a survey of the gain, resistance, dark noise, count rate capability, charge abstraction lifetime and image characteristics of a number of standard microsphere plates (of thickness 0.7 and 1.4 mm), operated both singly and as two-stage multipliers. Modal gains lay in the range 2.4–43 pC with pulse height relative FWHM values as low as 52%. MSP dark noise is relatively low (⋍0.5 counts cm 2 s −1 ) for well-chosen lower level discriminator settings. The image response is globally uniform, having local variations in image intensity on sub-mm scale, corresponding to the structure of MSP. Spatial resolution of an MSP detector may be better than 250 μm FWHM. No dependence of the gain on the angle of radiation incidence was observed for an angular range of 90°±25° (relative to the MSP surface), while the 2540 A UV relative detection efficiency varied by 10–15% over the same range. Lifetest measurements revealed that after removal of 8 × 10 −4 C cm −2 , the detector modal gain fell from 8 pC to half of its initial value. This fall in gain was compensated by raising the bias voltage.

MICROCHANNEL PLATE OPERATION AT HIGH COUNT RATES - FURTHER-STUDIES

In this paper, we extend and refine our previous model of the operation of microchannel plate (MCP) electron multipliers at high count rates [Fraser et al., Nucl. Instr. and Meth. A306 (1991) 247]. By analysing structured UV images obtained with a position-sensitive MCP detector, we confirm in detail the “pore bleaching” hypothesis of Anacker and Erskine [Rev. Sci. Instr. 62 (1991) 1246].

X-ray focusing using square-pore microchannel plates First observation of cruxiform image structure

Abstract Soft X-ray (0.28 and 1.74 keV) images produced by the focusing action of a planar, square-pore microchannel plate (MCP) are described. These images contain for the first time the three-component cruxiform structure expected from the studies of Angel [Astrophys. J. 233 (1979) 364] and Chapman et al. [Rev. Sci. Instr. 62 (1991) 1542], and so indicate a high degree of geometric regularity within and between the ∼ 46000 active channels in the MCP structure. The angular resolution of the central “true” focus is ∼ 10 arc min FWHM.

Microchannel plate detectors for /sup 14/C autoradiography

The authors describe a new type of detector for digital autoradiography which combines high sensitivity, useful detecting area (93/spl times/93 mm) and good spatial resolution (<100 /spl mu/m). This instrument is suitable, inter alia, for imaging beta-emitting whole body thin tissue slices. Measured detector sensitivity and linearity derived from images of a /sup 14/C standard are presented. The authors also describe results from (i) a /sup 14/C labelled rat whole body slice showing selective radiopharmaceutical uptake in clearly identifiable organs and (ii) /sup 14/C-Tamoxifen doped semi-thin tissue sections of rat womb and liver.

Dark noise in microchannel plate X-ray detectors

Abstract We present a detailed study of dark noise sources in microchannel plate (MCP) X-ray detectors. Previously postulated noise mechanisms are critically reviewed. Noise measurements carried out in the light of the review are then reported. The sea-level background count rate in two-stage MCP detectors is shown to have two principal components. The first — variable with position across the MCP and decaying with time under vacuum in a manner dependent on the plate history — is attributed to outgassing of the channel plate structure. The second — isotropic, independent of detector bias voltages, time and temperature — is shown to be consistent, in magnitude and in terms of its output charge spectrum, with the results of beta decay from the radioactive potassium content of the multiplier lead glass. Based on this identification of noise sources, prospects for the production of ultralow-noise MCP detectors for imaging X-ray astronomy are discussed. Measurements of noise reduction by coincidence methods using a partitioned anode are described.

Characterisation of a high resolution small field of view portable gamma camera

A handheld, high-resolution small field of view (SFOV) pinhole gamma camera has been characterised using a new set of protocols adapted from standards previously developed for large field of view (LFOV) systems. Parameters investigated include intrinsic and extrinsic spatial resolution, spatial linearity, uniformity, sensitivity, count rate capability and energy resolution. Camera characteristics are compared to some clinical LFOV gamma cameras and also to other SFOV cameras in development.

A large area MCP detector for X-ray imaging

Abstract We report on the X-ray performance of an imaging detector utilising a pair of 100 mm by 100 mm square microchannel plates. Gain and spatial uniformity are described, as are the imaging characteristics of the detector which uses a large format resistive anode readout. We also report on the uniformity of the quantum efficiency of the detector coated with a CsI photocathode. Background noise count rates are compared for 100 mm by 100 mm square microchannel plates manufactured from “standard” and radioisotope free glasses. Application of the large are detector to non-astronomy research, including imaging of “Lobster-Eye” type lenses and beta autoradiography are discussed.

A high resolution Small Field Of View (SFOV) gamma camera: a columnar scintillator coated CCD imager for medical applications

We describe a high resolution, small field of view (SFOV), Charge Coupled Device (CCD) based camera for imaging small volumes of radionuclide uptake in tissues. The Mini Gamma Ray Camera (MGRC) is a collimated, scintillator-coated, low cost, high performance imager using low noise CCDs. The prototype MGRC has a 600 μm thick layer of columnar CsI(Tl) and operates in photon counting mode using a thermoelectric cooler to achieve an operating temperature of - 10°C. Collimation was performed using a pin hole collimator. We have measured the spatial resolution, energy resolution and efficiency using a number of radioisotope sources including 140 keV gamma-rays from 99mTc in a specially designed phantom. We also describe our first imaging of a volunteer patient.